python regex only apply one replace

Question

I'm in-place editing a file using fileinput as below.

for line in fileinput.input():

    line = re.sub(pattern1, repl1, line)
    line = re.sub(pattern2, repl2, line)
    print(line, end="")

I want to apply re.sub only once for each line. If first pattern matches and replaced, I don't need to inspect pattern2.

How do I codify it?

RootTwo · Accepted Answer · 2016-02-14 03:03:28Z

1

Use re.subn, it returns the text and the number of subs made

for line in fileinput.input():

    line, n = re.subn(pattern1, repl1, line)
    if not n:
        line, n = re.sub(pattern2, repl2, line)
    if not n:
        line, n = re.sub(pattern3, repl3, line)

    ...

    print(line, end="")

Or if you have many patterns:

for line in fileinput.input():

    for pattern in patterns:
        line, n = re.subn(pattern, repl1, line)

        if n:
            break

    print(line, end="")

edited Feb 14, 2016 at 3:03

answered Feb 14, 2016 at 2:58

RootTwo

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Comments

heemayl · Accepted Answer · 2016-02-14 03:14:28Z

0

You can use re.search upfront to check if pattern1 exists and then make decision based on that:

for line in fileinput.input():

    if re.search(pattern1, line):
        line = re.sub(pattern1, repl1, line)
    else:
        line = re.sub(pattern2, repl2, line)

    print(line, end="")

answered Feb 14, 2016 at 3:14

heemayl

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